1Until the 4th century the popes were usually known only as bishops of Rome.2The higher number is used if Felix (II), who reigned from 355 to 358 and is ordinarily classed as an antipope, is counted as a pope.3Though elected on March 23, 752, Stephen (II) died two days later, before he could be consecrated, and thus is ordinarily not counted. The issue has made the numbering of subsequent Stephens somewhat irregular.4Either Leo VIII or Benedict V may be considered an antipope.5A confusion in the numbering of popes named John after John XIV (reigned 983-984) resulted because some 11th-century historians mistakenly believed that there had been a pope named John between antipope Boniface VII and the true John XV (reigned 985-996). Therefore they mistakenly numbered the real popes John XV to XIX as John XVI to XX. These popes have since customarily been renumbered XV to XIX, but John XXI and John XXII continue to bear numbers that they themselves formally adopted on the assumption that there had indeed been 20 Johns before them. In current numbering there thus exists no pope by the name of John XX.6Sylvester III is considered an antipope by those who believe that Benedict IX's forcible removal in 1044 was illegitimate.7In the 13th century the papal chancery misread the names of the two popes Marinus as Martin, and, as a result of this error, Simon de Brie in 1281 assumed the name of Pope Martin IV instead of Martin II. The enumeration has not been corrected, and thus there exist no Martin II and Martin III.